(1) Field of the Invention
The present invention relates to a high pressure metal vapor discharge lamp. More particularly, the present invention relates to a high pressure sodium lamp having an improved starting circuit.
(2) Description of the Prior Art
A known high pressure sodium vapor discharge lamp typically has the following structure.
More specifically, electrodes are attached to both the ends of an arc tube consisting of an alumina ceramic pipe to seal both the ends of the arc tube, and sodium, mercury and an inert gas are confined in the arc tube and the arc tube is arranged within an outer envelope.
In the early stage, a discharge lamp including Xe gas as the inert gas confined therein under about 20 Torr was used. Although this discharge lamp has a relatively high luminous efficacy of about 120 lm/w, the voltage necessary for starting the lamp is as high as about 4500 V and therefore, an exclusive ballast including a pulse generator arranged therein should be used as a starting circuit.
Then, a discharge gas including Ne-Ar (that is, penning gas) as the inert gas confined therein under about 20 Torr to reduce the starting voltage was developed. The luminous efficacy of this discharge lamp is relatively low and is about 100 lm/w. However, the voltage necessary for starting the lamp is as low as about 250 V and an expensive ballast need not be used as the starting circuit.
However, if it is intended to use conveniently this discharge lamp as a substitute of a 200 V high pressure mercury lamp customarily used, the starting voltage is still too high and an inclusive ballast should inevitably used as the starting circuit.
As means for moderating this disadvantage, there was proposed a starting aid for lowering the starting voltage (see, for example, U.S. Pat. No. 4,037,127). According to this proposal, a starting aid conductor such as a metal is mounted on the periphery of an arc tube adjacently or contiguously to the outer wall of the arc tube, and the conductor is connected to a lead-in line communicating with one electrode through a thermal switch and is brought close to the other electrode maintained at a potential opposite to that of said one electrode, whereby the starting voltage of the discharge lamp is lowered. By using this starting aid, it was possible to lower the starting voltage to about 160 V. This value means that the discharge lamp can be sufficiently started by a customary ballast for the conventional 200 V high pressure mercury lamp.
Recently, however, development of a discharge lamp having a much improved luminous efficiency has been desired, and as the discharge lamp satisfying this desire, there was proposed a discharge lamp including Xe gas confined therein under about 350 Torr (see Japanese Patent Application Laid-Open Specification No. 129468/78). The luminous efficacy of this discharge lamp is very high and is about 140 ml/w. However, the starting voltage of this discharge lamp is very high and is in the range of 8000 to 10000 V, and only by application of a starting aid, this discharge lamp cannot be started by a customary ballast of the conventional 200 V mercury vapor discharge lamp.